Change jQuery selector in Quick Edit to avoid losing data in IE8.

fixes #24923 for trunk.
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Restore the $current_site global in wp-admin/admin-header.php to avoid an undefined function error in single site.

see #25158.
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Merge the sticky menu component from MP6. The admin menu is now fixed if the viewport is large enough. props tollmanz, tillkruess, dd32. see #25858.

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Inline documentation for hooks in wp-includes/capabilities.php.

props ptahdunbar, kpdesign.
fixes #25531.
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3.8-alpha-26127 to bust all the caches.

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